Criticise son not on time (5)
I believe the answer is:
slate
'criticise' is the definition.
(I've seen this before)
'son not on time' is the wordplay.
'son' becomes 's'.
'not on time' becomes 'late' (I've seen this before).
's'+'late'='SLATE'
(Other definitions for slate that I've seen before include "Grey stone; tab" , "Metamorphic rock that can be split into thin layers" , "Bluish-grey colour" , "Stone used for roofing" , "Grey metamorphic rock" .)